The attractions of wave energy have been widely asserted. In summary, ocean waves offer potentially cheap renewable energy with limited emissions of climate-changing carbon dioxide.
However, large-scale commercial application of wave energy has proved elusive due to:                Low efficiency of capture and conversion        High lifetime cost        Vulnerability to extreme storm conditions        Unacceptable environmental impact.        
A solution described by Gregory (U.S. Ser. No. 12/884,792; 2010) is a dynamically tunable wave energy converter (WEC) arranged to pitch on ocean swell and comprising a dynamically tunable vessel enclosing a dynamically tunable compact gravity pendulum. The WEC is tuned to resonance with the dominant swell. The relative motion of vessel and pendulum mass is captured and converted to electricity. The WEC is sealed from the marine environment with no significant vulnerable external moving parts and is configured to allow in-situ repair. It can be submerged to avoid severe storms. Advantages claimed for this design are:                High efficiency of wave energy conversion due principally to use of the pitching vector and resonant energy transfer;        Low lifetime cost due to avoidance of sea-bed structures and reduced costs of repair and downtime;        Robustness against storms;        Unobtrusiveness.        
A suitable pendulum for this application is:                Dynamically (ie continuously and rapidly) tunable, allowing resonant energy transfer from swells of varying frequency;        Tunable over most of the frequency range of energetic ocean swells: typically 5 to 14 seconds period;        Compact ie of significantly smaller dimensions than an equivalent conventional pendulum at the longer periods of oscillation of typical energetic ocean swells (A conventional pendulum requires a distance from pivot to center of mass that varies with the square of the period, so that at long periods, the pendulum becomes impractically large: for example, 14 seconds period requires a structure that accommodates rotation over a radius greater than 50 m.)        Isochronous ie with a period of oscillation independent of the amplitude of the path of the pendulum mass: this enables control in conditions of varying wave height. Over deflections from the vertical of less than around 40% a pendulum mass following a circular path is almost isochronous. A mass following a cycloidal path is isochronous at all amplitudes.        
Such a pendulum is suitable for harvesting oscillating energy of variable frequency and amplitude at any scale.
Descriptions of pendulums in WECs date from the end of the 1800s. For example, Pitts (U.S. Pat. No. 613,075; 1898) describes a WEC using relative motion between a rocking float and an arm connected to a large mass suspended along the axial axis of the float and below it. The system is not tunable.
Conventional vertical pendulums in WECs have been described, sometimes in the form of pendulums that are able to swing in any vertical plane ie 360-degree pendulums. Conventional pendulums cannot resonate with the longer periods of high-energy swells without obtrusively and expensively large structures. Conventional vertical pendulums in WECs are described in the following patents:
Hoff (U.S. Pat. No. 656,645; 1900)
Gehre (U.S. Pat. No. 686,602; 1901)
Neal (U.S. Pat. No. 851,549; 1907)
Farmer (U.S. Pat. No. 974,869; 1910)
Lilley (U.S. Pat. No. 1,545,504; 1925)
Last (U.S. Pat. No. 3,696,251; 1972)
Filipenco (U.S. Pat. No. 3,912,938; 1975)
Marken (U.S. Pat. No. 4,438,343; 1984)
Beane (US 2011/0185719)
Hobdy (US 2010/0123313)
A variant of the conventional vertical pendulum is the inverted pendulum: this is also impractical for long period oscillation. Examples are Jacobi (U.S. Pat. No. 4,423,334; 1983), French (US 2004/0007880) and Smushkovitch (GB 2,436,644; 2007).
There are recurring descriptions of horizontal pendulums in WECs. These are usually configured as eccentrically weighted horizontally mounted wheels. Such pendulums can be compact. But the resonant period of a horizontal pendulum is infinite. This means that the pendulum will be randomly in or out of phase with vessel movements. An out-of-phase relative movement subtracts, rather than adds, energy to the pendulum. The random oscillation of the horizontal pendulum suits it to use only in random wave conditions where tunable inertial systems would be pointless, for example in chaotic, choppy seas. Its inefficiency makes it useless for commercial power production. Examples of horizontal pendulums in WECs, usually intended for low power applications such as signal buoys, are:
Singer and Wood (U.S. Pat. No. 624,490; 1894)
Keddy (U.S. Pat. No. 1,442,478; 1923)
Hegenbart (U.S. Pat. No. 1,584,293; 1925)
Hincke (U.S. Pat. No. 3,231,749; 1966)
Harding (U.S. Pat. No. 3,774,048; 1973)
Griffith (U.S. Pat. No. 4,256,971; 1981)
Ng (U.S. Pat. No. 4,266,143; 1981)
Slonim (U.S. Pat. No. 4,340,821; 1982)
Stupakis (U.S. Pat. No. 4,843,250; 1989)
Hench (US 2008/0093858):
Tracked pendulums in WECs have been described: these comprise a pendulum mass moving on a tracks with a vertical radius. A tracked pendulum can be compact, since neither pendulum arm nor pivot are used.
Caille (U.S. Pat. No. 721,501; 1908) describes an air pump driven by a mass sliding on a track in response to the rolling motion of a ship. Rahm (U.S. Pat. No. 1,494,804; 1924) describes a WEC using a mass moving on a track along the length of a ship, driven by pitching, and transmitting motion to a spirally grooved shaft. However, in both these cases the track is flat and so the period of oscillation is undefined. French (2004/0007880) describes a sliding mass in a WEC but the track is fixed: the radius of motion is fixed and therefore so is the frequency of oscillation. Beane (US 2011/0185719) describes a tracked mass in a WEC: the ends of the track are curved but the overall curvature of the track is not defined to be circular or cycloidal, so the tracked mass is not isochronous nor is it clear how the track radius might be dynamically adjusted.
Both French and Beane describe tuning by variable power take-off, or phase-forcing. For example, if the tracked mass is moving too fast (ie the period of oscillation of the mass is less than the period of the incident ocean swell), more power is extracted. A fatal defect in this method is that it requires reliable dynamic prediction of the power being transferred into the mass by the incident swell. This input of power depends on incident swell height, which in general is the result of interference between multiple wave trains and so is difficult to predict from moment to moment. By contrast, tuning by adjustment of the radius of the path of the mass can be substantially isochronous ie independent of wave height, enabling efficient resonant transfer of energy. Phase-forcing usually involves temporarily stopping or ‘latching’ of the mass, at which time no useful energy transfer takes place.
Tuning by latching is used by Clement (U.S. Pat. No. 7,989,975; 2011) who describes a WEC with a vertical pendulum. Clement's pendulum will usually be out of phase with the incident swell so that energy transfer is inefficient. By using latching, quasi-resonance is achieved and efficiency of energy transfer is improved but will typically be only a small fraction of the efficiency of resonant energy transfer.
Gregory describes two methods of dynamically tuning a tracked pendulum by varying the radius of the path of a tracked pendulum mass:                Using a flexible beam as the track and flexing the beam (Gregory GB 0916518.4; 2009)        Translating the pendulum mass between two fixed paths (Gregory US 2011/0089689)        
Gregory also describes tracked pendulums in WECs made isochronous at large amplitudes using cycloidally curved tracks (Gregory GB 1103510.2; 2011).
Folding pendulums in WECs have been described by French (US 2004/0007880) and by Gregory (GB 0916518.4; 2010). Additional linkages are used to control the path of an otherwise conventional vertical pendulum, so that a folding pendulum can be a compact pendulum. The methods used by French and Gregory differ.
In summary, excepting the tracked and folding pendulums already described by Gregory, described pendulums in WECs do not meet all of the following criteria:                Dynamically tunable        Tunable over the range of periods that characterize energetic ocean swell        Compact        Isochronous        
The present invention describes a pendulum that meets these criteria but is neither a tracked nor a folding pendulum.
A pendulum that is applied within a WEC to generate commercial levels of power requires large masses, in the range 10-1000 tonnes. Such a pendulum requires means of suspension that are reliable for long periods and also requires means of dealing with very large sideways forces. In the case of the tracked pendulum, dynamic adjustment of the track radius under very large loads is problematic and sideways forces place a major strain on the track. In the case of variants of the folding pendulum, similar concerns arise. An objective of the present invention has been to use robust means of suspension and in an embodiment of the invention, to deal with sideways forces by means of a compliant mechanical arrangement.